The indoor unit of an air conditioning system, which often is referred to as an air handler, typically includes a cabinet having at least a filter section, a heat exchanger section and a blower section. In electrically powered heating/cooling systems, such as heat pumps, the air handler usually also includes an electric heating section having one or more electric heating elements. The heating section is usually located downstream of the blower section in relation to the direction of air flow in the cabinet. One such air handler is shown and described in U.S. Pat. No. 3,977,467.
In operation, air to be cooled or heated is drawn into the cabinet through a return air duct and is first passed through a filter in the filter section to remove dirt and other debris. After the air passes through the filter, it flows across a heat exchanger coil in the heat exchanger section, which communicates with the suction side of an air blower in the blower section. The blower then discharges the air through the electric heating section into a supply duct that communicates with an indoor space. When the heating/cooling system (e.g., a heat pump system) is operated in a cooling mode and a vapor compression refrigerant is used as the heat transfer fluid, the heat exchanger coil functions as an evaporator to cool the air that flows through the heat exchanger section by vaporization of the refrigerant in the heat exchanger tubes. When the system is operated in a heating mode, the heat exchanger coil functions as a condenser to heat the air by condensation of the refrigerant. The electric heating elements are typically used at the beginning of a heating cycle and may also be used during the heating cycle to supplement the heating provided by the heat exchanger coil.
It is also known in the art to provide an elongated electrically resistive heating element in the blower section. Such heating element is permanently affixed in the blower section and is at least partially wrapped around the blower. Although this configuration eliminates the need for a dedicated electric heating section downstream of the blower section, the heating element is not field-replaceable to accommodate different electrical heating capacities.